Fatigue crack growth in ECAPed commercially pure UFG copper

AbstractThis work presents the experimental characterization of the fatigue crack growth resistance of an ultrafine-grained (UFG) copper alloy with a purity level equal to 99.90%. The UFG copper has an average grain size of 300 nm obtained by a 8-passes ECAP process throughout route Bc. The crack propagation behavior is investigated by standard fatigue crack propagation tests conducted in air, at load ratio R=Kmin/Kmax varying from 0.1 to 0.7, on Disk Shaped CT specimens. The tests are conducted at stage I and stage II regime of crack growth rate. Results are partially in contrast with the few experimental data available in literature about this material. In fact, the present copper shows a relatively high fatigue crack resistance with respect to the conventional coarse-grained alloy, especially when increases the applied ΔK=Kmax−Kmin. The analysis of some fracture surface morphologies corresponding to different growth rates, is conducted to highlight the propagation mechanism. A diffuse crack deflection and branching is observed at high nominal R-ratio, that could explain the crack retardation. However, a better understanding is needed on the effective role of the grain size, of the dislocations density and of the impurities at the grain boundaries. A micromechanical model based on a statistically equivalent microstructure obtained with a Voronoi tessellation is under study by the author in order to clarify these aspects